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Re: [wg14/wg21 liaison] Multidimensional subscript operator

From: Jₑₙₛ Gustedt <jens.gustedt_at_[hidden]>
Date: Fri, 30 Apr 2021 15:52:25 +0200

on Fri, 30 Apr 2021 11:59:40 +0300 you (Ville Voutilainen
<ville.voutilainen_at_[hidden]>) wrote:

> On Fri, 30 Apr 2021 at 11:47, Jₑₙₛ Gustedt <jens.gustedt_at_[hidden]>
> wrote:
> > My personal experience when talking to people from the C++ community
> > is that often there is a lack of such understanding. For example, to
> > know that VLA and VM types are different things, that VM types as
> > function interfaces don't have the disadvantages that have automatic
> > VLA if defined on the stack, and that VM types can easily be
> > dynamically allocated with no fuzz.
> >
> > Discussion about VM types (or VLA because people don't know the
> > difference) quickly become hostile. It seems that historically the
> > subject is quite overloaded. I would be cool if we could overcome
> > that social difficulty.
> I would kindly suggest that we start by you explaining by what you
> mean by "VM type". :)


In the C standard "VM type" stands for "variably modified type" and
comprises VLA and pointers to VLA.

In contrast to VLA themselves, pointers to VLA need only an amount of
storage that is fixed a compile time, so no stack overflow or other
nasty things like that can happen.

They are ABI compatible with usual pointers to arrays. That is
particularly interesting for function parameters of VLA type that decay
to such pointers, anyhow. Here an extended syntax (compared to C++)
allows to specify dynamic bounds that are evaluated in the scope of
defined function, and that allows (with some restrictions) the transfer
and verification of such bounds from the caller to the callee.

Another interesting application are dynamic allocations as Martin had
shown in one of the initial posts in this thread

   double (*x)[M] = malloc(sizeof(double[N][M]));
   x[i][j] = 0.;

Here, `x` can be used as a 2D matrix where the compiler can do all
index calculations without problems.

So where VLA themselves can be questioned for their inherent danger
for stack overflow, pointers to VLA are safe and provide good
opportunities for bound checking and efficient interfaces.

For an increased compatibility between C and C++, adding pointers to
VLA to C++ would be a winner, because that would make a lot of
numerical code portable between the two languages.

If C++ wouldn't allow the declaration of VLA, fine, maybe C could even
remove that possibility. I personally wouldn't mind much. (In C we
would need better integer constant expressions for the array bounds
before that, though.)


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Received on 2021-04-30 08:54:36